Buck boost converter confusion
Inducktion, Wed Apr 03 2019, 11:09PMHi all
Trying to understand this application circuit from the LTC1625 datasheet.
I understand they're trying to do synchronous rectification with the P channel fet to increase efficiency, but my question is...
Would it still work with higher output voltages? Say, 30 volts max?
I know right off the bat the gate drive IC would become very unhappy with 30 volts input; obviously, one would disconnect it and feed it straight 12 volts or something as an example.
But, otherwise, would the P channel fet still turn on? P channel mosfets have always confused me and to this day I still don't quite understand their operation.
Appreciate the help.
Re: Buck boost converter confusion
DerAlbi, Wed Apr 03 2019, 11:26PM
P-Fets are actually like N-Fets, just the opposite. If you know how to turn on a light bulb with an N-Fet, then built the same circuit with the P-Fet, and connect the supply voltage in reverse
Everything will work.
Regarding your question, i think it can be solved at system level without talking about the circuit:
At 12V output voltage, loosing 0.6V over a diode is, right there, a 5% systematic efficiency loss. So synch. rectification makes sense to get back some efficiency.
At 30V output voltage, this does not matter so much anymore (0.6V = 2% efficiency hit)
So, i would say, dont care if this works at 30V because it does not make sense to use a mosfet then anyway.
But you are correct, if you would leave the P-Fet in, it would not really work in that exact circuit. Actually, the LTC1693 dies first, since it only can live below 13.2V. So this limits your output voltage here.
DerAlbi, Wed Apr 03 2019, 11:26PM
P-Fets are actually like N-Fets, just the opposite. If you know how to turn on a light bulb with an N-Fet, then built the same circuit with the P-Fet, and connect the supply voltage in reverse
Everything will work.Regarding your question, i think it can be solved at system level without talking about the circuit:
At 12V output voltage, loosing 0.6V over a diode is, right there, a 5% systematic efficiency loss. So synch. rectification makes sense to get back some efficiency.
At 30V output voltage, this does not matter so much anymore (0.6V = 2% efficiency hit)
So, i would say, dont care if this works at 30V because it does not make sense to use a mosfet then anyway.
But you are correct, if you would leave the P-Fet in, it would not really work in that exact circuit. Actually, the LTC1693 dies first, since it only can live below 13.2V. So this limits your output voltage here.
Re: Buck boost converter confusion
Inducktion, Thu Apr 04 2019, 12:12AM
Is it worth synchronous rectification even with 10 amps output?
With 10 amps, over a 0.6 v drop schottky, gives about 6 watts of power dissipated.
With 10 amps through a 0.02 ohm PFET, that gives about 2 watts of power dissipated, plus the added complexity of the circuit...
There's another application circuit without synchronous rectification.
Should I just stick with this version, non-sync and not deal with the gate driver nonsense?
Inducktion, Thu Apr 04 2019, 12:12AM
Is it worth synchronous rectification even with 10 amps output?
With 10 amps, over a 0.6 v drop schottky, gives about 6 watts of power dissipated.
With 10 amps through a 0.02 ohm PFET, that gives about 2 watts of power dissipated, plus the added complexity of the circuit...
There's another application circuit without synchronous rectification.
Should I just stick with this version, non-sync and not deal with the gate driver nonsense?
Re: Buck boost converter confusion
DerAlbi, Thu Apr 04 2019, 12:33AM
You can scale that to any current you want, it will always remain the same argument. The efficiency impact is proportional to the voltage drop in relation to the overall output voltage. No matter if you draw 1A or 30A. In your example, if you assume 30V, 10A, then you deliver 300W. Do you really care about 4W savings somewhere? Even 2W on a component will most likely need some form of cooling (at least consideration). Once you are there, you can also cool the 6W
I cant really give more advice without knowing your constraints. I would try to reduce complexity. Its hard enough. So yeah, go for a diode for a start.
DerAlbi, Thu Apr 04 2019, 12:33AM
You can scale that to any current you want, it will always remain the same argument. The efficiency impact is proportional to the voltage drop in relation to the overall output voltage. No matter if you draw 1A or 30A. In your example, if you assume 30V, 10A, then you deliver 300W. Do you really care about 4W savings somewhere? Even 2W on a component will most likely need some form of cooling (at least consideration). Once you are there, you can also cool the 6W
I cant really give more advice without knowing your constraints. I would try to reduce complexity. Its hard enough. So yeah, go for a diode for a start.
Re: Buck boost converter confusion
2Spoons, Thu Apr 04 2019, 05:06AM
To complement what DerAlbi is saying, synchronous rectification might be worthwhile if you absolutely have to have max efficiency - maybe because you are running on a battery or other limited power source, or maybe that extra 4W is something you just can't get rid of easily.
Horses for courses, as usual.
2Spoons, Thu Apr 04 2019, 05:06AM
To complement what DerAlbi is saying, synchronous rectification might be worthwhile if you absolutely have to have max efficiency - maybe because you are running on a battery or other limited power source, or maybe that extra 4W is something you just can't get rid of easily.
Horses for courses, as usual.
Re: Buck boost converter confusion
Inducktion, Thu Apr 04 2019, 05:19AM
Gotcha. I'll keep it simpler. Gonna do some low side drivers for the "low" side fets (the fets I have are higher gate capacitance than the datasheet says the SMPS chip can handle on its own), and just use a low V-drop Schottky.
Which leads me to two more questions;
1. Why do really low v-drop schottkys have absolutely abysmal reverse leakage current? There were some I was looking at, that when "hot" would leak several hundred milliamps of current!
2. To "add" constant current to this, would it realistically be as simple as just adding a differential amp and a comparator, something like this?
The potentiometer sets the current "limit" by comparing the amplified voltage to some reference. Is there anything I should be careful of when doing this?
Again thank ya'll for your insight.
Inducktion, Thu Apr 04 2019, 05:19AM
Gotcha. I'll keep it simpler. Gonna do some low side drivers for the "low" side fets (the fets I have are higher gate capacitance than the datasheet says the SMPS chip can handle on its own), and just use a low V-drop Schottky.
Which leads me to two more questions;
1. Why do really low v-drop schottkys have absolutely abysmal reverse leakage current? There were some I was looking at, that when "hot" would leak several hundred milliamps of current!
2. To "add" constant current to this, would it realistically be as simple as just adding a differential amp and a comparator, something like this?
The potentiometer sets the current "limit" by comparing the amplified voltage to some reference. Is there anything I should be careful of when doing this?
Again thank ya'll for your insight.
Re: Buck boost converter confusion
DerAlbi, Thu Apr 04 2019, 07:14AM
Modifying the feedback path is not a fun thing to do. The amount of gain you bring in with a comparator will likely lead to oscillation- If you really want to do this, only use the OpAmp, so amplify the shunt voltage to the nominal FB-voltage at your target current. But you should 2 things to help to stabilize the regulation loop:
a) add a resistor between the output of the OpAmp and the FB-Pin. Then connect a capacitance between the output voltage and the FB-Pin. If there is a steep output voltage change, the regulation can work without phase delay from the OpAmp.
b) Your OpAmp adds gain to the feedback loop. Its bandwidth will require limiting . so add a capacitor parallel to the feedback resistor from Out to the inverting input.
----
In all honesty, such modification is BS
Just use a constant current driver, if you need a constant current. LED-Drivers are the way to go.
You still have not really communicated your requirements. But if i only go for the keywords ("buck boot, current regulation) then i would recommend the LT3755 for example. The easiest circuit would be the sepic-configuration depicted at page 17 of the data sheet. Not good for high power however.
DerAlbi, Thu Apr 04 2019, 07:14AM
Modifying the feedback path is not a fun thing to do. The amount of gain you bring in with a comparator will likely lead to oscillation- If you really want to do this, only use the OpAmp, so amplify the shunt voltage to the nominal FB-voltage at your target current. But you should 2 things to help to stabilize the regulation loop:
a) add a resistor between the output of the OpAmp and the FB-Pin. Then connect a capacitance between the output voltage and the FB-Pin. If there is a steep output voltage change, the regulation can work without phase delay from the OpAmp.
b) Your OpAmp adds gain to the feedback loop. Its bandwidth will require limiting . so add a capacitor parallel to the feedback resistor from Out to the inverting input.
----
In all honesty, such modification is BS
Just use a constant current driver, if you need a constant current. LED-Drivers are the way to go.
You still have not really communicated your requirements. But if i only go for the keywords ("buck boot, current regulation) then i would recommend the LT3755 for example. The easiest circuit would be the sepic-configuration depicted at page 17 of the data sheet. Not good for high power however.
Re: Buck boost converter confusion
johnf, Thu Apr 04 2019, 08:46AM
Induktion
you need to look up PID control (proportional integral differential)
this is how gain and phase shift are managed in real world systems mechanical to electronic it doesnt matter.
As for synchronous rectification as others have said it rapidly losses favour as the output voltage goes over 12 volts and would not be used at 48 volts output
johnf, Thu Apr 04 2019, 08:46AM
Induktion
you need to look up PID control (proportional integral differential)
this is how gain and phase shift are managed in real world systems mechanical to electronic it doesnt matter.
As for synchronous rectification as others have said it rapidly losses favour as the output voltage goes over 12 volts and would not be used at 48 volts output
Re: Buck boost converter confusion
Inducktion, Thu Apr 04 2019, 04:14PM
I see. Well, my requirements:
I'm trying to design an actually decent buck boost module that one could tack on a 12v supply and get good regulation, current capability and reliability. Constant current is a good thing to have which is why I'm trying to add it here. 0 to 30 volts with 10 amps current capability is what I'm shooting for
I know Chinese modules exist that do this sort of thing but I wanted to give designing one myself a try. It's relaxing to layout pcbs and actually try to make something new.
Inducktion, Thu Apr 04 2019, 04:14PM
I see. Well, my requirements:
I'm trying to design an actually decent buck boost module that one could tack on a 12v supply and get good regulation, current capability and reliability. Constant current is a good thing to have which is why I'm trying to add it here. 0 to 30 volts with 10 amps current capability is what I'm shooting for
I know Chinese modules exist that do this sort of thing but I wanted to give designing one myself a try. It's relaxing to layout pcbs and actually try to make something new.
Re: Buck boost converter confusion
Patrick, Thu Apr 04 2019, 06:12PM
Yes it can be.
Beware of PID loops which are dependent on microprocessors. They can work but if theres an isolation boundary, you can get a 6 volt oscillation on a 12 volt regulated supply thats stable and wont go away... but only magically appears with special load conditions you cant expect. so PID loops can be out-smarted. Thats why they're rarely used. Theres a lot of industry pressure to use PIDs to eliminate the whole Vector Network Analyzer phase of development.
Control loops and SMPSs are a lot of fun for me. but no small matter either. I recommend a feedback loop without isolation and no wall sockets for you. I have to admit a 450 volt 330uf cap bursting with sparks and steam is fun though. (if you still have a house.)
Patrick, Thu Apr 04 2019, 06:12PM
Inducktion wrote ...
I know Chinese modules exist that do this sort of thing but I wanted to give designing one myself a try. It's relaxing to layout pcbs and actually try to make something new.
I know Chinese modules exist that do this sort of thing but I wanted to give designing one myself a try. It's relaxing to layout pcbs and actually try to make something new.
Yes it can be.
Beware of PID loops which are dependent on microprocessors. They can work but if theres an isolation boundary, you can get a 6 volt oscillation on a 12 volt regulated supply thats stable and wont go away... but only magically appears with special load conditions you cant expect. so PID loops can be out-smarted. Thats why they're rarely used. Theres a lot of industry pressure to use PIDs to eliminate the whole Vector Network Analyzer phase of development.
Control loops and SMPSs are a lot of fun for me. but no small matter either. I recommend a feedback loop without isolation and no wall sockets for you. I have to admit a 450 volt 330uf cap bursting with sparks and steam is fun though. (if you still have a house.)
Re: Buck boost converter confusion
DerAlbi, Thu Apr 04 2019, 07:04PM
I would suggest to try LTSpice at this point. If you want CV/CC regulation this becomes very complicated very fast. If you want good regulation, i would suggest a linear output stage with a slower regulating switching pre-regulation. Maybe buy the Chinese module, see how it performs and what you wish to be better, then you have a better specification to verify in Spice.
DerAlbi, Thu Apr 04 2019, 07:04PM
I would suggest to try LTSpice at this point. If you want CV/CC regulation this becomes very complicated very fast. If you want good regulation, i would suggest a linear output stage with a slower regulating switching pre-regulation. Maybe buy the Chinese module, see how it performs and what you wish to be better, then you have a better specification to verify in Spice.
Re: Buck boost converter confusion
Inducktion, Thu Apr 04 2019, 07:43PM
This design is actually inspired by a Chinese module that uses a knockoff LTC1775/1625; biggest difference in mine though is that I'm using dedicated gate drive, bigger inductor and high side current regulation vs low side.
The Chinese module works, but like I said I want to make this my own. I'll try LTspice and see how things go.
Inducktion, Thu Apr 04 2019, 07:43PM
This design is actually inspired by a Chinese module that uses a knockoff LTC1775/1625; biggest difference in mine though is that I'm using dedicated gate drive, bigger inductor and high side current regulation vs low side.
The Chinese module works, but like I said I want to make this my own. I'll try LTspice and see how things go.
Re: Buck boost converter confusion
2Spoons, Thu Apr 04 2019, 09:36PM
Your current limit needs to be analog feedback : ditch the comparator, it has way too much gain so is likely to make the loop unstable. You want to feed back the current signal in such a way that it takes over from the voltage feed back - sort of an analog version of an "OR"gate.
I've attached a conceptual schematic - its basically two precision rectifiers connected so that "feedback" will be the higher of V sense or I sense. You will, of course, have to figure out all the phase compensation yourself.
2Spoons, Thu Apr 04 2019, 09:36PM
Your current limit needs to be analog feedback : ditch the comparator, it has way too much gain so is likely to make the loop unstable. You want to feed back the current signal in such a way that it takes over from the voltage feed back - sort of an analog version of an "OR"gate.
I've attached a conceptual schematic - its basically two precision rectifiers connected so that "feedback" will be the higher of V sense or I sense. You will, of course, have to figure out all the phase compensation yourself.
Re: Buck boost converter confusion
2Spoons, Thu Apr 04 2019, 09:40PM
I would attach a conceptual schematic - but I can't work out how to do that now.
2Spoons, Thu Apr 04 2019, 09:40PM
I would attach a conceptual schematic - but I can't work out how to do that now.
Re: Buck boost converter confusion
Inducktion, Thu Apr 04 2019, 11:34PM
Would using one of these
be out of the question?
Getting rid of the shunt to measure current would be a huge boon, since at the range of currents I'm asking for it'd dissipate too much power, or the op amp would have to have so much gain that I'd fear for stability/accuracy.
It only has 80 kHz bandwidth, so I assume that'd be too slow but still want to pose it as an idea. There are faster versions available as well.
Also, 2spoons, you can take screenshots by pressing the windows key + shift + S, and click and drag across what you want to snip, then go to snag.gy to paste it there and post a link to the image.
Inducktion, Thu Apr 04 2019, 11:34PM
Would using one of these
be out of the question?
Getting rid of the shunt to measure current would be a huge boon, since at the range of currents I'm asking for it'd dissipate too much power, or the op amp would have to have so much gain that I'd fear for stability/accuracy.
It only has 80 kHz bandwidth, so I assume that'd be too slow but still want to pose it as an idea. There are faster versions available as well.
Also, 2spoons, you can take screenshots by pressing the windows key + shift + S, and click and drag across what you want to snip, then go to snag.gy to paste it there and post a link to the image.
Re: Buck boost converter confusion
2Spoons, Fri Apr 05 2019, 12:50AM
Well yes you could use one of those, or you could use a 10mOhm resistor and more gain on your sense amp.
The noise figure for that allegro device is a bit crap. Its equivalent to about 100mA of noise signal. Which is ok if you are looking at 10A signals, but not for a 1A signal.
You can do better with a resistor and an op-amp, and you don't need isolation.
2Spoons, Fri Apr 05 2019, 12:50AM
Well yes you could use one of those, or you could use a 10mOhm resistor and more gain on your sense amp.
The noise figure for that allegro device is a bit crap. Its equivalent to about 100mA of noise signal. Which is ok if you are looking at 10A signals, but not for a 1A signal.
You can do better with a resistor and an op-amp, and you don't need isolation.
Re: Buck boost converter confusion
Inducktion, Fri Apr 05 2019, 01:33AM
Oh, I didn't even read the noise figure. Jesus. That's actually pretty bad, and looking at other similar parts they've all got a pretty nasty noise floor of about 100 mA.
Okay.... so.... what about something like this?
I lashed together a very crude buck converter in falstad's simulator (I know it's not LTspice, and I know the limitations of the sim, i.e. doesn't include parasitics etc)
Edit: (realized I was controlling this in a very goofy manner)
The op amp series I'm considering has an incredibly low offset voltage, of only 25 uV.
I know this is spitballing at this point, but I want to know your opinions before I go and actually order PCB's and parts for this.
Inducktion, Fri Apr 05 2019, 01:33AM
Oh, I didn't even read the noise figure. Jesus. That's actually pretty bad, and looking at other similar parts they've all got a pretty nasty noise floor of about 100 mA.
Okay.... so.... what about something like this?
I lashed together a very crude buck converter in falstad's simulator (I know it's not LTspice, and I know the limitations of the sim, i.e. doesn't include parasitics etc)
Edit: (realized I was controlling this in a very goofy manner)
The op amp series I'm considering has an incredibly low offset voltage, of only 25 uV.
I know this is spitballing at this point, but I want to know your opinions before I go and actually order PCB's and parts for this.
Re: Buck boost converter confusion
2Spoons, Fri Apr 05 2019, 03:47AM
Pretty much typical for current sense in the ground line.
2Spoons, Fri Apr 05 2019, 03:47AM
Pretty much typical for current sense in the ground line.
Re: Buck boost converter confusion
Inducktion, Fri Apr 05 2019, 06:13AM
My work thus far
Inducktion, Fri Apr 05 2019, 06:13AM
My work thus far
Re: Buck boost converter confusion
hen918, Fri Apr 05 2019, 11:40AM
You've got some good design features in there, like nice big power planes, but I think the power components should be closer together, to minimise inductance and make mounting more convenient. they could all be on the same side then you only need one heatsink
hen918, Fri Apr 05 2019, 11:40AM
You've got some good design features in there, like nice big power planes, but I think the power components should be closer together, to minimise inductance and make mounting more convenient. they could all be on the same side then you only need one heatsink
Re: Buck boost converter confusion
2Spoons, Sat Apr 06 2019, 12:24AM
For lowest EMI keep the switched node as small as possible. In the application example you posted that would be the junction of M1,M2 and L1.
Also planes are not a panacea for all ills - remember "current flows in loops" so you should always consider the return path of every signal. Sometimes you might want to steer a noisy or large current away from something sensitive, like your current sense amplifier. This can be done by adding slits to the plane in appropriate places.
2Spoons, Sat Apr 06 2019, 12:24AM
For lowest EMI keep the switched node as small as possible. In the application example you posted that would be the junction of M1,M2 and L1.
Also planes are not a panacea for all ills - remember "current flows in loops" so you should always consider the return path of every signal. Sometimes you might want to steer a noisy or large current away from something sensitive, like your current sense amplifier. This can be done by adding slits to the plane in appropriate places.
Re: Buck boost converter confusion
Inducktion, Sat Apr 06 2019, 12:43AM
Can you show me an example of what I can do to improve the design?
Does running ground traces around sensitive things help at all?
Inducktion, Sat Apr 06 2019, 12:43AM
Can you show me an example of what I can do to improve the design?
Does running ground traces around sensitive things help at all?
Re: Buck boost converter confusion
2Spoons, Sat Apr 06 2019, 02:07AM
It would help if you post an annotated schematic /pcb so we can see what is where.
The thing to watch out for is the ground plane isn't the same voltage everywhere if there are large currents flowing. So if your feedback needs to be referenced to the 0V leg of the controller chip then your sense amps need to be referenced to that point too, which means you dont want large currents flowing in the copper between the two - you might be better to run a separated ground from the controller to the sense amps.
Running a gnd trace around a sensitive area may be helpful in some cases, but it depends on the nature of the interference you are trying to stop.
2Spoons, Sat Apr 06 2019, 02:07AM
It would help if you post an annotated schematic /pcb so we can see what is where.
The thing to watch out for is the ground plane isn't the same voltage everywhere if there are large currents flowing. So if your feedback needs to be referenced to the 0V leg of the controller chip then your sense amps need to be referenced to that point too, which means you dont want large currents flowing in the copper between the two - you might be better to run a separated ground from the controller to the sense amps.
Running a gnd trace around a sensitive area may be helpful in some cases, but it depends on the nature of the interference you are trying to stop.
Re: Buck boost converter confusion
Inducktion, Sat Apr 06 2019, 06:12AM
Here!
I made a schematic and tried annotating the PCB layout a little bit. Link is to a gallery with both shown.
All of the part numbers shown on the schematic image line up with the ones on the PCB; i.e. R5 is R5, C1 is C1, etc.
I know I'm not the best at making schematics so I apologize, but I hope it's readable and understandable.
One other big question, is am I suppose to have my "bulk" capacitance before or after the sense resistor? Or should I realistically have bulk capacitance before AND after?
Additionally, some of the values of components aren't shown; they're likely going to be figured out experimentally.
Inducktion, Sat Apr 06 2019, 06:12AM
Here!
I made a schematic and tried annotating the PCB layout a little bit. Link is to a gallery with both shown.
All of the part numbers shown on the schematic image line up with the ones on the PCB; i.e. R5 is R5, C1 is C1, etc.
I know I'm not the best at making schematics so I apologize, but I hope it's readable and understandable.
One other big question, is am I suppose to have my "bulk" capacitance before or after the sense resistor? Or should I realistically have bulk capacitance before AND after?
Additionally, some of the values of components aren't shown; they're likely going to be figured out experimentally.
Re: Buck boost converter confusion
2Spoons, Sat Apr 06 2019, 09:16AM
I would put your bulk C before the Isense, otherwise you will be measuring the ripple current from the inductor as well.
Your CV/CC implementation wont work - you have both V and I feedback running to a summing junction - which is not what you want. You need the greater of V or I to control the regulator, not the sum. You need to join the two via precision rectifiers.
I would also allow for a gate resistor on each mosfet - you may not need them, but if you do then having the footprint on the pcb will make life much easier.
You really ought to try simulating the circuit before you build anything. At least do the CC/CV bit.
This is what I mean for the CC/CV combining:
2Spoons, Sat Apr 06 2019, 09:16AM
I would put your bulk C before the Isense, otherwise you will be measuring the ripple current from the inductor as well.
Your CV/CC implementation wont work - you have both V and I feedback running to a summing junction - which is not what you want. You need the greater of V or I to control the regulator, not the sum. You need to join the two via precision rectifiers.
I would also allow for a gate resistor on each mosfet - you may not need them, but if you do then having the footprint on the pcb will make life much easier.
You really ought to try simulating the circuit before you build anything. At least do the CC/CV bit.
This is what I mean for the CC/CV combining:
Re: Buck boost converter confusion
Inducktion, Sat Apr 06 2019, 08:53PM
Does it need to be a precision rectifier, or would just using two low Vdrop diodes by themselves work?
And I actually do have spaces for gate resistors on two of the fets already, so that's taken care of. I'll add one for the last one too.
And I'll also move the bulk capacitance before the sense resistor.
and I did actually try to do a simulation on LTspice with it, but the output didn't make sense and the entire thing ran INCREDIBLY slowly. It feels very archaic for some reason.
Oh, and should I have the feedback for CV be after or for the sense resistor too? Or does that not overly matter?
Inducktion, Sat Apr 06 2019, 08:53PM
Does it need to be a precision rectifier, or would just using two low Vdrop diodes by themselves work?
And I actually do have spaces for gate resistors on two of the fets already, so that's taken care of. I'll add one for the last one too.
And I'll also move the bulk capacitance before the sense resistor.
and I did actually try to do a simulation on LTspice with it, but the output didn't make sense and the entire thing ran INCREDIBLY slowly. It feels very archaic for some reason.
Oh, and should I have the feedback for CV be after or for the sense resistor too? Or does that not overly matter?
Re: Buck boost converter confusion
Inducktion, Sat Apr 06 2019, 11:24PM
Updated the PCB. Moved the current sense amp down, farther away from the switching node. Made special care to try to avoid any power ground/signal ground loops with it too.
Added two rectifiers from the CV and CC feedback nodes to the main feedback loop. They're D5 and D7.
Also moved the bulk output cap to a new spot, and added another one for more filtering, and added another plane so the two sides of the PCB conduct current for it.
Added more vias and fixed some layout issues regarding the ground planes; they're more continuous and don't pinch down in any specific spot.
Added a gate resistor to M1.
Inducktion, Sat Apr 06 2019, 11:24PM
Updated the PCB. Moved the current sense amp down, farther away from the switching node. Made special care to try to avoid any power ground/signal ground loops with it too.
Added two rectifiers from the CV and CC feedback nodes to the main feedback loop. They're D5 and D7.
Also moved the bulk output cap to a new spot, and added another one for more filtering, and added another plane so the two sides of the PCB conduct current for it.
Added more vias and fixed some layout issues regarding the ground planes; they're more continuous and don't pinch down in any specific spot.
Added a gate resistor to M1.
Re: Buck boost converter confusion
2Spoons, Sun Apr 07 2019, 01:02AM
The reason I suggest using a precision rectifier is the temp coefficient of ordinary diodes will screw up your output voltage / current as they heat or cool. A dual opamp for this job is pretty cheap ( it doesn't need to be anything fancy - just unity gain stable) and makes the problem just go away.
I would put Vsense right on the output terminals of your supply, because thats the voltage you are trying to control.
I did a quick and dirty on the power section to show you a more compact way to lay out the heart of the converter. This minimises all the current path lengths, and noisy node areas, and puts all the semis on one side for a single heatsink as suggested by Hen918. The yellow is where the top and bottom layers overlap - its a bit ugly, but you'll get the idea. I haven't tried to route out the gate signals - I wouldn't want to spoil your fun.
I know the sizes of the parts will be wrong but its just conceptual.
2Spoons, Sun Apr 07 2019, 01:02AM
The reason I suggest using a precision rectifier is the temp coefficient of ordinary diodes will screw up your output voltage / current as they heat or cool. A dual opamp for this job is pretty cheap ( it doesn't need to be anything fancy - just unity gain stable) and makes the problem just go away.
I would put Vsense right on the output terminals of your supply, because thats the voltage you are trying to control.
I did a quick and dirty on the power section to show you a more compact way to lay out the heart of the converter. This minimises all the current path lengths, and noisy node areas, and puts all the semis on one side for a single heatsink as suggested by Hen918. The yellow is where the top and bottom layers overlap - its a bit ugly, but you'll get the idea. I haven't tried to route out the gate signals - I wouldn't want to spoil your fun.
I know the sizes of the parts will be wrong but its just conceptual.
Re: Buck boost converter confusion
Inducktion, Sun Apr 07 2019, 02:18AM
Ah okay!
That makes sense.
I squeezed in a basic dual op amp right next to the LTC1775;
Uses an LM2904, really cheap and "internally compensated with unity gain" ? I assume that means it's unity gain stable.
Edit: Just gonna use an LM358 instead. They're a fraction cheaper and from what I read they're unconditionally unity gain stable.
Also uses a really cheap dual common cathode signal diode, so all in all adds a whole 47 cents to the cost. Not bad. If it adds more stability then great.
And I do understand the mounting on one heatsink thing, however I'm kind of smitten with the layout I have... I hope it doesn't cause any issues but we'll see. Plus I actually like being able to use individual heatsinks. I have small ones that work perfectly.
The layout I have is actually pretty similar to the chinese version I'm loosely basing this off of.
Inducktion, Sun Apr 07 2019, 02:18AM
Ah okay!
That makes sense.
I squeezed in a basic dual op amp right next to the LTC1775;
Uses an LM2904, really cheap and "internally compensated with unity gain" ? I assume that means it's unity gain stable.
Edit: Just gonna use an LM358 instead. They're a fraction cheaper and from what I read they're unconditionally unity gain stable.
Also uses a really cheap dual common cathode signal diode, so all in all adds a whole 47 cents to the cost. Not bad. If it adds more stability then great.
And I do understand the mounting on one heatsink thing, however I'm kind of smitten with the layout I have... I hope it doesn't cause any issues but we'll see. Plus I actually like being able to use individual heatsinks. I have small ones that work perfectly.
The layout I have is actually pretty similar to the chinese version I'm loosely basing this off of.
Re: Buck boost converter confusion
2Spoons, Sun Apr 07 2019, 05:28AM
If you like your layout, then go for it. Engineering is as much art as science.
I'm used to being constrained by things such as budgets or EMC standards, so I tend to optimize for those things automatically - that's what 30 years in the game does to you!
2Spoons, Sun Apr 07 2019, 05:28AM
If you like your layout, then go for it. Engineering is as much art as science.
I'm used to being constrained by things such as budgets or EMC standards, so I tend to optimize for those things automatically - that's what 30 years in the game does to you!
Re: Buck boost converter confusion
Inducktion, Sun Apr 07 2019, 08:31AM
Thank you very much for your insight and help with this by the way.
I've ordered everything, and I'll update this thread once I get everything together and can test!
Inducktion, Sun Apr 07 2019, 08:31AM
Thank you very much for your insight and help with this by the way.
I've ordered everything, and I'll update this thread once I get everything together and can test!
Re: Buck boost converter confusion
Inducktion, Wed Apr 10 2019, 12:38AM
I uh...
I had no internet last night so I spent the day making a new layout.
Also added a small 7 volt regulator down at the bottom. I think this design is a lot cleaner and also gives more room between components.
Inducktion, Wed Apr 10 2019, 12:38AM
I uh...
I had no internet last night so I spent the day making a new layout.
Also added a small 7 volt regulator down at the bottom. I think this design is a lot cleaner and also gives more room between components.
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